Combined manual and automatic gain control circuit



June 7, 1960 D. D. HOLMES 2,939,950

COMBINED MANUAL ANO AUTOMATIC GAIN CONTROL CIRCUIT Filed July I, 1957 INVENTOR. DAVID D. HOLMES B755 United States Patent O COMBINED MANUAL AND AUTOMATIC GAIN CONTROL CIRCUIT David D. Holmes, Princeton, NJ., assignor to Radio Corporation of America, a corporation of Delaware Filed July 1, 19,57, serial No. 669,108 s Claims. (ci. 25o-20) The present invention relates to radio receivers and particularly to improved gain control circuits for suchreceivers. While not restricted thereto, the invention is particularly useful in direction finder receivers.

An automatic gain control (AGC) circuit tends to maintain the signal output of a receiver at a constant level for wide ranges of input signal. This is disadvantageous in direction-finder receivers. Here, low amplitude signals must be detected in order to obtain highly accurate null indications, and automatic gain control circuits may tend to reduce the receiver sensitivity (gain) somewhat, even at low input signal levels. If the AGC circuits were removed, strong input signals would severely overload the receiver and cause distortion or even obliteration of the receiver output signal.

It is an object of the present invention to provide an improved automatic gain control circuit which overcomes the diliiculties outlined above.

Another object of thevinvention is to provide an improved, transistorized automatic gain control circuit.

Yet another object of the invention is to provide an improved automatic gain control circuit which includes an automatic self-defeating feature.

The automatic gain control circuit of the present invention is rendered operative only in response to the tendency of the receiver to be driven into overload. At other times, the gain control circuit is automatically defeated. The circuit which defeats the AGC action includes a switch such as a diode in the gain control circuit which is maintained conductive in response to severe overload of the receiver but which is at other times driven beyond cut-off. The parameters sensed by the defeat circuit to determine receiver overload include the receiver volume control setting, the level of signal passing through the receiver, and the receiver gain.

The invention will be described in greater detail by reference to the following description taken in connection with the accompanying drawing in which:

Figure l is a block and schematic circuit diagram of a portion of a superheterodyne radio receiver showing the AGC circuit; and,

Figure 2 is a graph to explain the operation of the circuit of Figure 1.

The superheterodyne receiver, shown in part in the drawing, includes an antenna, radio frequency ampliiier, converter and several IF stages. These are shown -by a single block 8. Stage 10 is the final intermediate frequency amplifier and includes a transistor 12. 'I'he output of the transistor is coupled via transformer 14 to the second detector 16, shown as a diode. The derector is direct-coupled to the audio driver 20, shown as a transistor. The audio driver is connected to a pair of transistors 24, 26 connected in complementary symmetry and forming the output stage. These transistors drive a loudspeaker 28. Meter 30 is for indicating a null.

The portions of the circuit described so far operate in a straightforward manner and need not be explained in detail. Transistors 12, 20 and 24 are PNP type, however, NPN type can be used instead, with minor circuit modiiication, as will be understood by those skilled in the ICC Volume control potentiometer 32 is connected through resistor 34 to the negative terminal of battery 36. The

slider 38 of the potentiometer is connected to two resistance networks. The first 42, 44, leads to the base 46 of audio driver stage 20. The second, which includes resistor 48, leads to common bus 50 to the bases of the transistors comprising the iirst intermediate amplifier, radio frequency amplifier, and converter. The AGC voltage, which is developed at point 52, passes through AGC latch-on diode 54, when the latter conducts, to the gain control bus 50.

In operation, assume that it is desired to increase the receiver gain. This may be done by making the bases of the PNP transistors more negative. Adjustment of potentiometer 32 in a sense to decrease the resistance it inserts accomplishes this.

When the base 46 of the audio driver 20 is made more negative, the driver draws more current and emitter 56 becomes more negative. The emitter is connected through resistors S8, 60 and 62 to the anode 64 of di'ode 54. The anode therefore tends to become more negative and the diode tends to be driven further beyond cut-off. However, potentiometer 32 is also connected through resistor 48 to the cathode 66 of diode 54. Therefore, as the receiver gain is increased, cathode 66 becomes more negative and this tends to drive diode 54 into conduction. Since both anode and cathode are driven more negative by the action described, the two effects tend to balance.

When the gain of the receiver is increased, the output signal level of the final IF stage increases. The IF output signal'is applied to the second detector 16. The latter is so poled that increased signal drives base 46 moreV positive and this, in turn, causes the transistor to conduct less current and emitter 56 to be driven more positive. A portion of the signal developed at emitter 56 is iiltered by the resistor condenser network 62, 67 and applied to anode 64. Thus, the increased signal from the intermediate frequency stages, due to the increased receiver gain, causes anode 64 of diode 54 to tend to become more positive (in actuality, less negative). This tendency more than compensates for the tendency of anode 64 to be driven negative due to the bias applied to audio driver 20 by volume control potentiometer 32. Since4 cathode 66 is made more negative in increasing the receiver gain and, at the same time, anode 64 is made less negative by the increased signal applied to second detector 16, diode 54 is driven toward conduction.

The values of the circuit components described are such that any tendency of the receiver to overload to more than a predetermined extent causes the diode 54 to conduct and the AGC voltage at point 52 to be applied via bus 50 to the early receiver stages. On the other hand, when either the received signal level or the receiver gain is reduced, anode 64 becomes more negative than cathode 66, the diode 54 is cut-olf, and the AGC action is defeated.

One can think of the above action in the following way. Therev are three parameters which must be taken into account to determine when diode 54 is driven into conduction. One is the volume control setting which, in this circuit, affects the receiver gain; the second is receiver gain; and the third is the incoming signal level. Reducing the receiver gain tends to defeat the AGC action. Reducing the input signal strength tends to de. feat the AGC action. Increasing the receiver gain tends to re-establish the AGC action. However, in a preferred' form of the invention, the circuit values are so chosen that even with the volume control adjusted for maximum receiver gain, thevAGC action is defeated if `the received;

signalis of suflicientlylowamplitude 4(a rnull indication, Y Yfor example).

Figure 2 shows, in a qualitative way, the mode of operation of the circuit. Each of curves 70, 71 and 72 repren sentsa different volume'controlsetting. Notethateven Vat maiiimum gain; the AGC circuit is defeated for input signals of lower than a predetermined level.` The .level 73 at which diode 54 is rendered conductive lis higher than Vthat required to'foverload the receiver. This :level canYreadily be adjusted by :adjustment of the "circuit parameters (for example, the values-of the resistors in thevoltage divider networks-)'Vand, in ffact,rmay even be made lowerthan' the overload level shown-by dashed line'74. Y

In direction finder receiver applications, it is desirable to increase receiver output volume when approaching a nullV indication in order'to obtain a sharp transition.Y `If this were done in a receiver Without-an AGC circuit, the Vaudio driver output signal on eac'hpside ofthe null would be of suicient amplitude severely to overload the receiver. With the 'present circuit, however,V yas soon as the receiver tends to overload beyondagiven point, diode 54 is rendered conductiveriand AGCV action isestablished. What is claimed is: 1,-A gain control Vcircuit for a radio receiver cornprising, in combination, a gain control circuit extending Y from oneV of the final stages of the receiver to theY control element of at least one'of thefearlier stages of the re-V ceiver, said circuit including a diode so poled that when it conducts it permits a gain controlvoltage to pass through saiddiode to said control elementwof said earlier stage, one element of the diode being connected to said 'earlier stage of the receiver; a volumev control circuit in said receiver; circuit means connecting both` elements of said diode toV said volumecontrol circuit;l and vcircuit means connecting one of said elements of said diode to Ythe signal channel ofsaid receiver for sensing `the amplitude of a signal passing through said receiver, theV circuit constants of said two. circuit means being such that the diode is rendered conductive only in response to receiver overload of more than `a predetermined extent and rendered cut-off under other conditions.

2. .In `a radio receiver, jin combination, ,anaudio varnpliiier stage having ya controlelement to'which a bias may be applied, Vand a second clement the potentialof which lvaries as a function of said bias;,a gain controll circuit for sensing the voltage atsaid second element'and deriving Vtherefrom a gaincontrol voltagc;,;a. switch in said gain control circuit Ywhich inrone state` permits the gain i control voltage to pass andV inanother state prevents the same `from passing; Va .volume VcontrolV potentiometer con- Y nected to said controlV elementV for 'biasing saidV VAaudio amplifier, yand, connected kto said switch end tending-to placeV the same in saidone state in response to a settingY producing high gain andV inv itsjother. state in response toV Ya setting producing low gain; and circuit .means ,con-1 Ynccted tov saidV second element-and responsive rto ga" large amplitude Yinput vsignal-,tosa'idaudio'ampliier for tending to place .saidswitch in said one-state andtoalow amplitudesignal to said-audioampler for tending' YtoY Yplace said switch in its Vother state.. ,Y f

V 3. In a radiovreceiver, in combination, an audio ampliiier comprising a transistor having base, collector and Y s' n2,939,950

sive toa large amplitude input signal to said audio ampliiier for tending to place said switch in said one state and to a low 'amplitude signal to said Laud-io amplifier for tending to place said switch in its other state.

4. In the combination as `set-k forth in claim 3, said switch comprising Ia diode.

5. In Ia radio receiver, in combination, an audio stage including `an amplifying device having a control element, and a second element the .potential of which depends upon'the potential applied to said control element; a

etector coupled to said control element for applying ya voltage thereto which varies as an inverse function of the amplitude of the signal 4applied to the detector; an automatic .gain Vcontrolrcircuit connected to Vsaid second element for producing a gain control'voltage which de4 pends upon the value of the voltageY developed atlsaid second element; a volume control circuit; and a diode in said gain .control circuitV which,` when it fis'cut off,

blocks said gain control voltage,-on`e element of said diode being connected to the volume control circuit and the other elementof said diode beingfconnected lto said second element, the diode beingso poledV that chan-ge in theV volume control setting in -a sense to increase-the receiver output signal tends Vto render the'diode conductive, and increase in theA level of the signal Iapplied to YYsaid detector tendsrto rendersaid diode conductive.

`6. In a radio receiverasset forth --in claim Y5, said aniplify-ing device comprising a transistor, said control ele-1 ment comprising the base of-ksaid'transistor, and' said second element comprising the emitterV of said transistor. 7. In aV radio receiver, a plurality of receiver stages rat Vleast one of which includes -afcontrol' element Vto which a direct voltagemayV be applied yfor controlling theY gain of that stage; a volujrne control vcircuit connected to one of said stages for adjustingthe receiver gain; an output stage; and an automatic gainl control circuitconnected between said Voutput'sta'ge -and said control elements of said receiver stages and switching means in said .gain control circuit responsive both to the voltage outputof said volume control circuit'and the 'signal' outputA of=said output` stagefor selectively Iapplying a gain control: voltage tol said'control elements of said'receiver stages-only in response toa given relationship between said voltage output of said volumevcontrol circuit and said signal'output of said outputistage.

-8. In a radoreccivena pluralityy 'ofV receiver stagesat least some of which include a control element to which a directrvoltage may be applied for controlling the gain' ofi that stage; avolumeA control circuit connected to another of saidvstages forV adjusting the receiver gain; an output stage; and |an'auto'rnatic gain control circuit connected between saidoutput'stage iand said c'ontrolele ments of said .receiver stages-andswitchingimeansfin said gain control circuit which in one state'ren'd'ers saidcon'- trol circuitoperativeV and i-nanother state renders said control circuit. inoperative, Y said switching -means Vlbeing responsivevboth to the'voltag'e outputof said V,volume vcontrolfcircuitand'thesignaloutput Vof'said output stage emitter electrodes; again control circuit for sensing the,

voltage `at Vsaid emitter electrode andderiving therefrom again control voltage; a Vswitch in said gain controlcir cuit Whichin one state permits, the gain control voltage Y toipass land in another state Yprevents theV same Afrom passing; a volume control potentiometer connectedV to said base for biasing said audio amplifier, and connected tosaid switchjand tendingtoplace the same in said one state in. responseY to' a setting producing -high receiver Y "outputsignalrand'in its other state'in response toV a set- Y tingY producing llow Vreceiver Voutput signal; andv circuit u Y means connected to said'AY emitter electrode and respon- Y for `applyingia igain'controllvoltage'to said.v control elements ofsaid receiver stages-in response'to ya givefrelationship between said voltage output Yof said volume control circuit and said signalV output` ofsalidoutput stage.

YReferences itsd in theleof this patent'V` UNITED STATES MrrnN'rsY rGreat Britain V Dec. 28, 195e: 

